Magnetic tunnel junctions (MTJs) are devices with potential use as magnetoresistive read heads in magnetic storage applications, such as hard disk drives, and as nonvolatile storage cells for magnetic random access memory (MRAM) applications.
For both MTJ read head and MRAM applications a useful MTJ device is one comprised of two thin ferromagnetic layers separated by a very thin insulating layer, such as alumina (Al.sub.2 O.sub.3), in which one of the ferromagnetic layers is fixed or "pinned" by being exchange biased (also called "exchange coupled") to a thin layer of an antiferromagnetic material. In a MTJ read head, the moment of the other ferromagnetic layer, called the "free" layer, is oriented generally perpendicularly to the moment of the pinned ferromagnetic layer in zero magnetic field and is free to rotate in the presence of magnetic fields from the recorded media. In a MTJ cell, the cell is designed to exhibit two bi-stable states in zero magnetic field in which the moment of the free ferromagnetic layer is oriented either parallel or antiparallel to the moment of the pinned ferromagnetic layer. These states, which exhibit different tunneling resistance values when current is passed perpendicularly through the device, represent the two storage states of the MTJ memory cell. In a MTJ read head application the moment of the free ferromagnetic layer is oriented generally perpendicular to the moment of the pinned ferromagnetic layer in the absence of an applied magnetic field, and rotates about this position in the presence of applied fields from the magnetic recording medium.
It is important that MTJ devices withstand fairly high temperatures during processing, For MTJ MRAM applications, if complementary metal-oxide semiconductor (CMOS) electronic devices are used with the MTJ cells the highest processing temperature will be determined by details of the particular CMOS process used. It is likely that the MTJ cells will be fabricated after the CMOS circuits have been fabricated, within the "back-end-of-line" (BEOL) process where the temperatures to which the devices are subjected are more limited, but may still be as high as .about.400.degree. C. For MTJ read heads, processing temperatures in excess of 250.degree. C. are likely in order to hard-bake certain photoresists used in fabricating the read/write heads. The most common type of antiferromagnetic material proposed for use in MTJ devices is a Mn--Fe alloy. MTJ devices using Mn--Fe antiferromagnetic layers on a variety of underlayers have failed at temperatures as low as 250-300.degree. C. Even for moderate anneal temperatures as low as 250.degree. C., the MTJ devices using Mn--Fe antiferromagnetic layers are not highly thermally stable and typically the magnetoresistance of such devices is reduced. It is believed that during processing a small amount of Mn diffuses from the Mn--Fe layer to the interface between the pinned ferromagnetic layer and the alumina tunnel barrier, which reduces the Magnetoresistance of the device.
It addition to poor thermal stability, Mn--Fe has several other disadvantages, including a relatively low exchange coupling field, poor corrosion resistance, and a low blocking temperature (the temperature at which the net magnetic moment no longer has a fixed orientation).
Other antiferromagnetic materials besides Mn--Fe have been proposed for exchange biasing the pinned ferromagnetic layer in spin-valve type magnetoresistive read heads. For example, European published patent application EP-0717422 describes the use of Ir--Mn alloys, and suggests without any experimental data that Os can be one of 30 other elements (approximately 25% of the Periodic Table) to be added to the Ir--Mn alloy for the purpose of improving the corrosion resistance of the alloy. U.S. Pat. No. 5,552,949 suggests the use of X--Mn alloys, where X can be one of 10 elements, including Os, provided the X element is present in the range of 25 to 76 atomic percent in the X--Mn alloy. In both of these spin-valve head references the antiferromagnetic layer is deposited directly on top of the ferromagnetic layer.
What is needed is a MTJ device for read head and MRAM applications that uses an antiferromagnetic material that provides a high exchange coupling field and good corrosion resistance and results in a MTJ device that is thermally stable at high processing temperatures.